Footwear has been developed and become popular that has an outer sole bonded to a mid-sole that is bonded to a shoe upper. An insole is removably mounted in the shoe above the mid-sole. The mid-sole usually has at least one layer of molded cushioning foam elastomeric material that is bonded to the outer sole that has a bottom durable tread surface. The cushioning foam material initially provides adequate cushioning against impact forces during running and jumping. The outer sole material with its tread surface has been improved to such an extent that it often outlasts the cushioning properties of the cushioning foam material of the mid-sole. The inner sole may be replaced as needed.
During specific sports, the impact of compression on the outer sole is highly uneven with most of the compression impact forces occurring in a few concentrated areas of the shoe. As such, the cushioning properties of the foam material in the mid-sole can wear out certain areas of the shoe while the remainder of the shoe, particularly the tread surface and the shoe upper that is commonly made from leather or canvas material maintains a useful and cosmetically acceptable condition.
For example, in basketball, there is tremendous compressive and impact forces exerted on the ball section and the heel portion of the shoe. As such, the wear pattern on the outer sole is heavily concentrated on the ball section and heel section of the outer sole. If there is insufficient cushioning, risk of injury increases to the foot and or ankle. If the cushioning is too soft, it may be overly compressed, i.e. it may bottom out and create jarring on the foot and ankle thereby also increasing fatigue and risk of injury. If the cushioning is too stiff, it also provides unnecessary jarring to the foot and ankle. The level of cushioning ideally is different for different people. The main factors to determine the appropriate cushioning are the weight of the person and the intensity of the activity i.e. how high the person jumps or how fast he runs.
From the above description it becomes clear that it is advantageous for having a replaceable spring cushioning cassette that can be customized to provide optimum cushioning for the wearer and can be easily installed into the mid-sole of the shoe particularly at the areas of highest impact and wear. Secondly, it is also desirable to determine the compressive impact stresses exerted on the shoe to help determine the optimum cushioning cassette and maintain a consistent fit and cushioning resistance needed for the particular shoe and wearer.